The Fischer-Tropsch reaction converts a mixture of CO and hydrogen into hydrocarbons. There are two types of catalyst for the Fischer-Tropsch reaction: iron-based catalysts, which work optimally at a temperature on the order of 350° C. (called “high-temperature FT catalyst”), and cobalt-based catalysts, which work at a lower temperature, generally below 250° C. They are primarily comprised of an active phase and an oxide support. A summary of the prior art on catalysts for the FT reaction based on cobalt is provided in the article “On the selectivity of cobalt-based Fischer-Tropsch catalysts: Evidence for a common precursor for methane and long-chain hydrocarbons” by S. Lögdberg et al., published in 2010 in J. Catalysis (doi:10.1016/jcat.2010.06.007). The oxide supports consist primarily of alumina, silica and optionally titanium dioxide. The oxide supports have excellent properties making it possible to design active catalysts for the FT reaction, but they also suffer from disadvantages such as their very low thermal conductivity, low hydrothermal resistance, the presence of acid sites at the surface (alumina), low mechanical strength in particular for extrudates (silica and titanium dioxide) and low attrition resistance for microbeads used in a bubbling bed (in particular for silica and TiO2).
To increase the mechanical and hydrothermal stability of the oxide supports, it has been sought to modify these supports. Mention may be made of alumina promoted by lanthanum oxide La2O3 (U.S. Pat. No. 5,537,945 and U.S. Pat. No. 6,255,358 (Energy International Corp.), U.S. Pat. No. 7,163,963 (Conoco Phillips Co.)), by Si (U.S. Patent Publication No. 2005/0124490 (Chevron Texaco Corp.), U.S. Pat. No. 7,365,040 (Sasol Technology)) and by Ti or Zr (U.S. Pat. No. 6,975,7209 (Sasol Technology)). Spinel supports have also been proposed (patent applications WO 2006/067285 (Institut Français du Pétrole) and U.S. Patent Publication No. 2007/0161714), as well as amorphous silica supports (U.S. Patent Publication No. 2010/0311570) for increasing attrition resistance. The alumina used is generally predominantly alumina-γ, but alumina supports including a majority of alumina-α have also been used (U.S. Pat. No. 7,351,679 (Statoil ASA)).
It has been reported in the literature that TiO2 supports make it possible to produce extremely selective cobalt-based catalysts (C5+) for the FT (see the publications published in the journal Catalysis Today, vol. 100 (2005), p. 343-347 and in the Journal of Catalysis, vol. 236 (2005), p. 139, as well as in Applied Catalysis A: General, vol. 210 (2001) p. 137-150). However, the authors mention that their mechanical strength is too low.
TiO2 is generally used to increase interactions with particles of the active phase. It has been reported in the literature that a silica support covered with a layer of TiO2 would make it possible to increase the activity and selectivity of a silica support. The results obtained on this type of silica-based support promoted with TiO2 (see the publication “Influence of Support Preparation Methods on Structure and Catalytic Activity of CoTiO2—SiO2 for Fischer-Tropsch Synthesis” of S. Mu et al., published in 2009 in the journal Catal. Lett. 133, p. 341-345, and the publication of S. Hinchiranan et al. published in 2008 in the journal Fuel Proc. Technol. 89, p. 455-459) demonstrate a substantial improvement in the catalytic activity in FT synthesis. The introduction of TiO2 in the support also modifies the liquid hydrocarbon selectivity. Nevertheless, according to the deposition mode and the configuration of the FT test, fixed bed or bubbling bed, the influence is different, e.g. an improvement in the case of a FT test in fixed-bed mode and a slight drop in the case of a test in stirred-bed mode. In every case, the improvement in the catalytic activity in FT is attributed to a better dispersion of the cobalt particles with a smaller size in the presence of TiO2.
Finally, in tubular fixed-bed reactors, the use of catalysts supported on β-SiC makes it possible to temper the thermal variations in the catalytic bed owing to the high thermal conductivity of the β-SiC material. All of these advantages make it possible to perform the FT synthesis under more extreme conditions in order to improve the productivity of the method.
Nevertheless, a catalyst supported on a β-SiC material is less active than its homologs supported on oxides.
The present invention is intended to prepare a novel type of silicon carbide-based (SiC) support for the Fischer-Tropsch synthesis (FTS) reaction that has better stability and better efficiency as well as better selectivity.